The sister of a British dual national human rights activist held in a Cairo jail and on the 74th day of a hunger strike, on Tuesday urged the UK foreign secretary to publicly demand that Alaa Abd El Fattah is saved from death by being released.

Sanaa Seif was speaking at an event attended by Richard Ratcliffe, the husband of Nazanin Zaghari-Ratcliffe, the released British-Iranian dual national, and Gurpreet Singh, the brother of Jagtar Singh Johal – a Sikh activist from Dunbarton detained by the Indian police nearly five years ago. He has still not been charged and says a confession was extracted under torture.

Seif said of her brother: “He is slowly dying in his prison cell but he wants to live. Why is the British government not standing up to President al-Sisi? Even after top-level phone calls, my brother is still being refused British consular access.

“How can Sisi ignore Boris Johnson like this? Where is Liz Truss? Will nobody stand up to Egypt before my brother dies in prison? Why has Truss not yet made an official statement about him. I can tell you, it hurts.”

Her brother, perhaps Egypt’s best known dissident, is currently taking 100 calories a day after the relaxation of his prison conditions. Generally, the recommended daily intake for men is 2,500 calories. Another sister, Mona, who visited him last week, was so shocked at his deteriorating condition that she has started a solidarity strike.

Sanaa Seif said she had been advised by the Foreign Office not to go public as there were many open lines, but she felt that route was not working. Her brother is serving a five-year sentence for sharing a blog post about Egyptian prison conditions.

David Lammy, Mona Seif’s constituency MP and the shadow foreign secretary, called on Truss to meet the family. He said: “Britain as a nation has to decide where it stands on human rights,” adding that families had learned that staying quiet did not work.

He said the Foreign Office’s efforts at the highest level to release British nationals appeared “arbitrary, haphazard, sometimes uncoordinated, sometimes lacking resource and certainly lacking transparency”. He said he was worried about “a foreign policy approach that is entirely transactional and not values-based. If you approach foreign policy as entirely transactional you want diplomacy to be quiet and in the dark and not in the light.”

Lammy added: “The British state needs to go public. Where you have disagreements with partner nations with which you are friendly, you get more respect if you are clear about human rights issues; you can have that robust conversation and stand up for your own citizens.”

Ratcliffe said the Foreign Office offered sympathy but provided little traction. He said: “The British government is no longer very serious about torture.” He said that in 2011 William Hague, then foreign secretary, had produced guidelines on how to combat torture, and each year the Foreign Office recorded 100 torture cases a year – and yet in the past 20 years the only case where a foreign secretary had directly accused another government of the torture had been that of his wife.

He said Pakistan had been accused of 69 cases of torture during the period that it was the largest recipient of British aid.

Sacha Deshmukh, Amnesty International UK’s chief executive, said: “This has already gone on for far too long and we need the UK government to make Alaa’s case an absolute priority. It’s frankly bewildering that the UK has not yet even publicly called for Alaa’s release. Why not? What are they waiting for?”

The press conference heard from friends and relations of other British Foreign Office consular cases, including the former BBC journalist Peter Juvenal, arrested six months ago by the Taliban, and James Fitton, a 66-year-old man sentenced by an Iraqi court for 15 years for stealing archeological artefacts said to be no larger than a finger nail.